Thermodynamic Multi-Objective Optimization of an ORC-LNG Combined Cycle System

A thermodynamic model of a dual loop organic Rankine cycle (ORC) combined with liquefied natural gas (LNG) expansion system has been developed to analyze the thermodynamic performance for the purpose of recovering LNG cold energy and blast furnace slag water waste heat. Furthermore, a multi-objective genetic algorithm (GA) is employed to solve the optimal solutions from the viewpoints of maximizing thermal efficiency and exergy efficiency simultaneously over the whole operating range of the combined cycle system. The results show that the optimal evaporation temperature and the intermediate condensation temperature of the first ORC (cycle A) are 83 °C and 19 °C separately. The optimal condensation temperature of the secondary ORC (cycle B) is -63 °C . The optimal gasification pressure and the supply pressure of the LNG expansion are 3.92 MPa and 0.20 MPa. At the rated condition, the ORC-LNG combined cycle system has the maximum thermal efficiency of 46.12% and exergy efficiency of 51.67%, with a 4.39 MW total net power output.